The present invention relates to an improved circle drawing tool which is equipped with a doughnut-shaped outer board, a circular slide track mounted onto the outer board and an inner circular board provided with rolling balls in a plurality of receiving cavities thereof and having such a diameter that it can be rotarily engaged with the slide track as well as the doughnut-shaped outer board in assembly. On the doughnut shaped outer board are disposed a plurality of round holes distributed closely one by one consecutively according to their diameters. There are 4 straight lines separated apart at 90 degrees in angle. There are a number of small pen insertion holes consecutively spaced by 4 mm on each line and each starting point of the four lines is 1 mm farther away from the center of the outer board one after the other. There are 10 radial lines equally divided on the inner circular board. On each radial line there are a plurality of consecutive pen insertion holes defined thereon with every two neighboring points 10 mm spaced apart. Each starting point of all the 10 lines is 1 mm farther away in a consecutive order from the center of the circular board so as to make the pen insertion holes distributed in a certain range in 1 mm division level. An extension arm having a plurality of pen insertion holes can be removably connected to the rotary outer board so as to permit circles larger than the diameter of the outer board to be drawn.
A compass is the most common tool used by people to draw circles, but there are different auxiliary instruments developed to facilitate the drawing of circles of various diameters, such as a standard circle board on which circles of various sizes are orderly distributed with their diameters marked beside each hole so as to permit people to make circle drawings with ease. However, such prior art circle boards are limited in the size of the circles to be drawn and the number of holes on each board is also limited. There are other various prior art auxiliary circle drawing boards which are complex in structure and relatively expensive to produce.